In a conventional radio relay transmission method, a transmitter transmits a radio signal having a vertical polarization, for example, and a radio repeater receives the radio signal transmitted from the transmitter and amplifies the received radio signal to be transmitted as a radio signal having a vertical polarization toward a receiver. Thus the radio repeater amplifies the received radio signal and transmits it as a radio signal having the same polarization as the polarization of the received radio signal. Consequently, the receiver receives the radio signal having the same polarization as the polarization of the radio signal transmitted by the transmitter.
In such a radio repeater, a reception antenna and a transmission antenna thereof have an identical polarization characteristic. Consequently, a radio signal transmitted from the transmission antenna of the radio repeater is received in a loop interference way by the reception antenna of the repeater. A signal which is received in the loop interference way will be hereafter referred to as a loop interference signal. If an amplification gain of the radio repeater is increased when the repeater has a reception frequency equal to a transmission frequency, the presence of the loop interference signal causes an oscillation. This limits an increase in the amplification gain.
A technique which estimates and suppresses a loop interference signal in a radio repeater is proposed, for example, in H. Hamazumi et al, “A study of a ioop interference canceller for the relay stations in an SFN for digital terrestial broadcasting”, Global Telecommunications Conference 2000, GLOBECOM '00. IEEE, vol. 1, PR 167-171, 27Nov. 1 Dec. 2000 (hereafter referred to as literature 1). As illustrated in FIG. 1, for example, in a radio repeater equipped with a loop interference suppressor of the prior art, a radio signal received by a vertical polarization reception antenna 1 is fed through a loop interference suppressor 2 to an amplifier 3, and the radio signal amplified by the amplifier 3 is transmitted from a vertical polarization transmission antenna 4. The received radio signal from the reception antenna 1 is input to a loop interference channel estimator 5 where a transmission path characteristic (impulse response) of a transmission path 8 for the loop interference signal from the transmission antenna 4 to the reception antenna 1 is estimated. The estimated transmission path characteristic is set up in an FIR filter (finite impulse response filter) 6 where the transmission characteristic of the loop interference transmission path 8 is convoluted into the received radio signal from the recepetion antenna 1 to produce a replica of the loop interference signal. The replica is subtracted from the received radio signal from the reception antenna 1 in a subtractor 7 before it is input to the amplifier 3.
A multiple radio repeater which uses a composite antenna including a horizontal (or vertical) polarization antenna and a vertical (or horizontal) polarization antenna, one of which is used in receiving a transmitted wave from a mating station while using the other for transmission to a mating station to achieve a coupling between different polarizations which is equal to or greater than 40 dB for separation between the transmitted wave and the received wave is disclosed in Japanese Laid-Open Patent Application (26735/80) (hereafter referred to as literature 2). The reception antenna of this radio repeater has an antenna directivity pattern including a main beam of a narrow width which is chosen to be in the direction of a transmitter antenna.
A multiple input multiple output (MIMO) system in which a plurality of information series are transmitted from a radio transmitter on a common frequency band and the radio signals on the same frequency band are received by a receiver to be separated into respective information series is disclosed in Katsumi Sakai et. al, “Multipoint relay transmission system”, The Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE RCS 2001-263 (hereafter referred to as literature 3), which is shown in summary in FIG. 2. A transmitter 10 transmits M information series S1 . . . , SM (where M is an integer equal to or greater than 2) from transmission antennas AS1 . . . , ASM as radio signals having a vertical polarization on a common frequency band. M radio signals are received by L radio repeaters 201, . . . , 20L (where L is an integer equal to or greater than 1) and are once stored therein. Upon termination of transmission from the transmitter 10 such as one call, one burst or the like, in response to a temporary cease of transmission from the transmitter 10, the radio repeaters 201, . . . , 20L amplify the stored radio signals to form radio signals having a vertical polarization, which are transmitted toward a receiver 30. The receiver 30 include N linear polarization reception antennas Ar1, . . . , ArN (where N is an integer equal to or greater than M) which receive the radio signals from the radio repeaters. The received signals are subject to MIMO equalization processing to be separated into M information series S1 . . . , SM. If the transmitter 10 and the receiver 30 are located in a line of sight environment, the provision of the radio repeaters 20 allows the channel capacity to be improved by increasing transmission paths through the radio repeaters 20 in addition to linear transmission paths joining between the transmitter 10 and the receiver 30.